The invention relates to an asynchronous time-division multiplex transmission system comprising an interconnection element, which element transmits cells supplied by auxiliary lines and destined for a trunk line at a preset interconnection element transmission rate and which comprises cell filters coupled to each of the auxiliary lines, which filters pass the cells for storage in intersection buffers coupled to each of the cell filters when the path identification contained in the cells is allocated to the trunk line.
In the asynchronous time-division multiplex transmission system useful information, for example, telephone, picture or sound signals, is transmitted in fixed-length blocks over digital signal processing arrangements. A fixed-length block is understood to be a cell that has a preset number of bits in a serial order. Each cell comprises a header and an information field. The header field comprises, for example, the path identification for the cell. A path identification is understood to be a connection identification or path routing information. The connection identification comprises the data on the target or sub-target for the useful information. Within the system the path routing information is added in specific transmission arrangements and this information contains data on a sub-target in the transmission arrangement. The useful information is accommodated in the information field.
Cells are successively allocated to certain time intervals (time frames). The duration of such a time interval depends on the clock frequency employed for the transmission components. If no useful information is available, idle cells, i.e. cells without useful information, are transmitted in such a time frame. Cells that do contain useful information are denoted useful cells.
During the transmission of the cells among subscribers, the cells pass through switching networks in which paths are formed by means of path identification evaluation. Such a switching network is habitually composed of a plurality of switching network blocks. Such a switching network block which has a plurality of auxiliary lines and trunk lines is constituted by a plurality of interconnection elements. An interconnection element is connected to a plurality of auxiliary lines and a trunk line. In an interconnection element cells are passed from an auxiliary line to a trunk line. When cells arrive from a plurality of auxiliary lines during a time frame, which cells wish to access a trunk line, specific coupling strategies are necessary.
Aforesaid interconnection element is disclosed in patent application P 38 33 490 to which U.S. Pat. No. 5,067,124 corresponds. The cells then occurring on the auxiliary lines are stored in an intersection buffer for each line if a cell filter passes cells to the intersection buffer. In the cell filter the cell is stored in a register and by means of a comparator it is checked on the basis of the cell path identification and the trunk line address stored in the address memory whether the cell is allocated to the trunk line. If the cell is to be passed to the trunk line, it is stored in the intersection buffer. Cells that do not belong to this trunk line are checked in further interconnection elements. If the intersection buffers are released to be read out, they apply cells to the trunk line. Decisions on the order in which the cells are read out are made by an allocation circuit in which the cells, in the order in which they have been written, are released to be read out. If a plurality of cells have arrived simultaneously, they will be read out in a predetermined order. In an interconnection element of this type, various intersection buffers may be loaded more than other buffers. Since the intersection buffers have all equal storage capacity and the size of an intersection buffer is determined in such a way that no cells are lost in the case of high load, intersection buffers having a large storage capacity are necessary.